Nanocarbon solubilizer, method for purifying same, and method for producing high-purity nanocarbon

ABSTRACT

A water solubilizer for nanocarbons contains a surfactant which can form a spherical micelle vesicle having a diameter of 50-2,000 nm in a water solution or a water-soluble polymer having a weight-average molecular weight of 10,000-50,000,000 as an active constituent. For example, the water solubilizer is used for purification of nanocarbons.

This application is a U.S. National Phase Application under 35 USC 371of International Application PCT/JP03/15445, filed Dec. 2, 2003, whichis incorporated herein in its entirety by this reference.

TECHNICAL FIELD

The present invention relates to the art of easily obtaining high-puritynanocarbons from crude nanocarbon products, in particular, a nanocarbonsolubilizing agent (solubilizer) (for example, a form of an aqueoussolution capable of solubilizing carbon nanotubes), a process forrefining (purifying) nanocarbons using the same and a process forproducing high-purity nanocarbons.

BACKGROUND ART

Nanocarbon is a new material, represented by carbon nanotube, whichdraws attentions in various fields of energy, electronics, chemistry,pharmaceuticals, optics, materials and mechanics, etc. Carbon nanotubesinclude single- and multi-layered types and cup-stack types. The single-and multi-layered types are needle-like carbon molecules having adiameter in the order of nanometers and have a structure of graphenerolled into a cylinder. Those having a multi-layer structure composed ofconcentrically placed graphene cylinders are called multi-walled carbonnanotubes, while those consisting of a single layer of graphene cylinderare called single-walled carbon nanotubes.

Incidentally, there have conventionally been a number of proceduresprovided for refining crude carbon nanotube products. For example,Japanese Unexamined Patent Publication (Kokai) No. 2000-290008 disclosesa process for refining carbon tubes which comprises the first step ofdispersing a crude product containing carbon tubes into a gold colloidalsolution, the second step of removing a solvent from the gold colloidalsolution containing the crude product and the third step of heating thecrude product under oxygen atmosphere. This technique utilizes theprinciple that gold tends to act as a catalyst when rendered particulateand oxidize carbon at a low temperature.

In the technique as described in Japanese Unexamined Patent PublicationNo. 2000-290008, an example of solvents for the solution to be used iswater; however, the carbon nanotubes per se exist as dispersed in anaqueous solution without being dissolved.

In addition, the Preliminary Proceedings of 51st Symposium onMacromolecules, Oct. 2 to 4, 2002 at Kyushu Institute of Technology,Kitakyushu, Japan discloses a technique of solubilizing single-layeredcarbon nanotubes into water. This technique, however, solubilizes carbonnanotubes into water by surface-treating the carbon nanotubes with anamphipathic compound having a bilene group to impart hydrophilicity totheir surfaces. Thus, when the surfaces are modified, such modificationsmust be removed afterward depending on the application, which makes itquite cumbersome.

The present invention aims to provide a novel technique for solubilizingnanocarbons into water without modifying nanocarbon surfaces.

DISCLOSURE OF THE INVENTION

As a result of trial and error, the inventor has discovered thatnanocarbons may be solubilized by encapsulating the nanocarbons inglobular micelles (microsomes) or pseudomicelles and has attained thepresent invention. The term “pseudomicelle” as used herein refers to thestate formed by nanocarbons being wrapped around (for example, entwinedor enclosed) by a macromolecular surface active agent, in which asubstance wrapped around will as a whole exhibit hydrophilicity so thatit may be solubilized in a manner similar to the case with micelles.

Specifically, the present invention (1) is a water-solubilizing agentfor nanocarbons comprising, as an active ingredient, a surface activeagent capable of forming globular micelles having a diameter of from 50to 2000 nm in an aqueous solution (hereinafter referred to as “micelletype”) or a water-soluble macromolecule having a weight averagemolecular weight of from 10,000 to 50,000,000 (hereinafter referred toas “pseudomicelle type”).

The present invention (2) is the water-solubilizing agent according tothe invention (1) wherein the surface active agent is a phospholipid- ornon-phospholipid-based surface active agent.

The present invention (3) is the water-solubilizing agent according tothe invention (2) wherein the surface active agent is one or moreselected from the group consisting of distearoylphosphatidylcholine(DSPC), dimyristoylphosphatidylcholine (DMPC),dipalmitoylphosphatidylcholine (DPPC),3-[(3-cholamidopropyl)dimethylamino]-2-hydroxy-1-propanesulfonate(CHAPSO), 3-[(3-cholamidopropyl)dimethylamino]-propanesulfonate (CHAP)and N,N-bis(3-D-gluconamidopropyl)-cholamide.

The present invention (4) is the water-solubilizing agent according tothe invention (1) wherein the water-soluble macromolecule is avegetable-based surface active agent.

The present invention (5) is the water-solubilizing agent according tothe invention (1) wherein the water-soluble macromolecule is a compoundselected from water-soluble polylysacchride, such as alginates (forexample, alginic acid, propylene glycol alginates), gum arabic, xanthangum, hyaluronic acid, chondroitin sulfate, water-soluble cellulose, suchas cellulose acetate, methyl cellulose, hydroxypropyl methyl cellulose,chitosan, chitosan, chitin; water-soluble proteins, such as gelatin,collagen; polyoxythylene-polyoxypropylene block copolymer; and DNA.

The present invention (6) is the water-solubilizing agent according toany one of the inventions (1) to (5) which is in the form of an aqueoussolution.

The present invention (7) is the water-solubilizing agent according tothe invention (6) wherein the agent further comprises ananocarbon-permeating substance and an oxidizing agent and the pH rangesfrom 6 to 14.

The present invention (8) is the water-solubilizing agent according tothe invention (7) wherein the nanocarbon-permeating substance is lithiumion.

The present invention (9) is the water-solubilizing agent according tothe invention (7) or (8) wherein the oxidizing agent is a persulfate.

The present invention (10) is the water-solubilizing agent according toany one of the inventions (1) to (9) wherein the nanocarbons are carbonnanotubes (single- and multi-layered types and cup-stack types), carbonnanofibers or carbon nanohorns.

The present invention (11) is the water-solubilizing agent according toany one of the inventions (1) to (10) which is used for refiningnanocarbons.

The present invention (12) is a process for refining nanocarbonscomprising the step of adding a crude product containing nanocarbons tothe water-solubilizing agent as defined in any one of the inventions (6)to (11) in the form of an aqueous solution, thereby dissolving thenanocarbons into the water-solubilizing agent.

The present invention (13) is the process for refining nanocarbonsaccording to the invention (12) which further comprises the step oftreating the crude product containing nanocarbons with an acid beforeadding the crude product to the water-solubilizing agent when a metalcatalyst was used in a process for producing the crude product.

The present invention (14) is a process for producing high-puritynanocarbons comprising the step of adding a crude product containingnanocarbons to the water-solubilizing agent as defined in any one of theinventions (6) to (11) in the form of an aqueous solution, therebydissolving the nanocarbons into the water-solubilizing agent.

The present invention (15) is the process for producing high-puritynanocarbons according to the invention (14) which further comprises thestep of treating the crude product containing nanocarbons with an acidbefore adding the crude product to the water-solubilizing agent when ametal catalyst was used in a process for producing the crude product.

BEST MODE FOR CARRYING OUT THE INVENTION

First of all, water-solubilizing agents on the basis of the micelle typewill be described. Surface active agents to be used for this type arethose capable of forming globular micelles having a diameter of from 50to 2000 nm (preferably from 50 to 300 nm) in an aqueous solution.Reasons for the suitability of globular micelles (microsomes) of thesesizes are not clear; however, the following assumptions can be made atthe moment. For example, carbon nanotubes usually have a length in therange of from 100 to 1000 nm. When a water-solubilizing agent accordingto the present invention is used in the form of an aqueous solution, thecarbon nanotubes will be folded to a fraction of their length (forexample, to the order of one-fourth of their length) so that they mayhave a length of from several tens of nanometers to several hundreds ofnanometers in the aqueous solution. It is presumably understood that theabove sizes are appropriate for housing such folded carbon nanotubes inthe microsomes, with a result that the carbon nanotubes can effectivelybe solubilized. It is also assumed for other nanocarbons that they maybe housed in micelles according to the same mode of action.

There has previously been a technique in which a surface active agent isadded (Japanese Unexamined Patent Publication No. 2002-255528). Themicelles formed by this technique, however, are very small in the orderof 0.1 nm and the principle of the technique is such that carbonnanotubes will adhere to the surfaces of the micelles. The presentinvention is based upon the new concept that nanocarbons (for example,nanotubes) are housed within micelles (microsomes) instead of adheringto the surfaces of the micelles.

The term “globular micelle” (“microsome”) as used herein refers to amicelle formed by a surface active agent, which has a globular housingspace. For example, in the case of phospholipid-based surface activeagents, such microsomes are called liposomes. The diameter of theglobular micelle (microsome) represents a value as determined accordingto light scattering method (pH-unadjusted aqueous solution at 20° C.).

Surface active agents which may be used are not particularly limited aslong as they have the above-specified characteristics. For example, bothphospholipid-based surface active agents and non-phospholipid-basedsurface active agents to be subsequently referred to may be used.

The term “phospholipid-based surface active agent” here means anionicand twitterionic surface active agents having a phosphate group as afunctional group, which may be of either a phospholipid (including bothglycerophospholipid and sphingophospholipid) or a modified phospholipid(for example, hydrogenated phospholipid, lysophospholipid,enzyme-converted phospholipid, lysophosphatidylglycerol, complexes withother substances). Such phospholipids are found in various membranesystems of organism-composing cells, such as protoplasmic membranes,nuclear membranes, microsome membranes, mitochondrial membranes, Golgicell membranes, lysosomal membranes, chloroplast envelopes and bacterialcell membranes. Preferably, phospholipids used for liposome preparationare preferable. Specific examples may include phosphatidylcholines [suchas distearoylphosphatidylcholine (DSPC), dimyristoylphosphatidylcholine(DMPC) and dipalmitoylphosphatidylcholine (DPPC)],phosphatidylethanolamine, phosphatidylinositol, phosphatidylserine,phosphatidylglycerol, diphosphatidylglycerol, lysophosphatidylcholineand sphingomyelin.

The term “non-phospholipid-based surface active agent” means ionic andtwitterionic surface active agents not containing a phosphate group as afunctional group, examples of which may include3-[(3-cholamidopropyl)dimethylamino]-2-hydroxy-1-propanesulfonate(CHAPSO), 3-[(3-cholamidopropyl)dimethylamino]-propanesulfonate (CHAP)and N,N-bis(3-D-gluconamidopropyl)-cholamide.

Next, water-solubilizing agents on the basis of the pseudomicelle typewill be described. Water-soluble macromolecules to be used for this typeare those having a weight average molecular weight of from 10,000 to50,000,000 (preferably from 10,000 to 5,000,000). Weight averagemolecular weights here are based on values as determined by gelpermeation high performance liquid chromatography using pullulan as thestandard.

The above water-soluble macromolecules are not particularly limited aslong as they have the above-specified molecular weights, examples ofwhich may include compounds selected from various vegetable-basedsurface active agents, water-soluble polysaccharides, such as alginates,(for example, alginic acid, propylene glycol alginate), gum arabic,xanthan gum, hyaluronic acid, chondroitin sulfate, water-solublecelluloses, such as cellulose acetate, methyl cellulose, hydroxypropylmethyl cellulose, chitosan, chitin; water-soluble proteins, such asgelatin, collagen; polyoxyethylenepolyoxypropylene block copolymer; andDNA.

The water-solubilizing agents according to the present invention (themicelle and pseudo types) will then be described with respect to otherrequirement. First, the water-solubilizing agents according to thepresent invention are in the form of an aqueous solution during use.Nevertheless, any water-solubilizing agents which are brought into theform of an aqueous solution before use shall be included in the conceptof “water-solubilizing agent”, including not only aqueous solutions butalso liquid concentrates, kits divided into parts and dry types whichare brought into an aqueous solution before use.

For the micelle type, the content of a surface active agent must beequal to or higher than the critical concentration of the micellesforming microsomes when in the form of an aqueous solution. Usually, thecontent is from 0.2 to 10 mmol per liter of aqueous solution for 1 g ofcrude product. For the pseudomicelle type, the content of awater-soluble macromolecule is not particularly limited. Usually, thecontent is from 5 to 50 g per liter of aqueous solution for 1 g of crudeproduct.

It is preferable that the water-solubilizing agent according to thepresent invention further comprises a nanocarbon-permeating substanceand an oxidizing agent and is in the form of an aqueous alkalinesolution. This preferred embodiment will then be described below.

First, the term “nanocarbon-permeating substance” means a substancehaving a diameter which is smaller than the C—C lattice size of ananocarbon. A nanocarbon-permeating cation having such a diameter (iondiameter), specifically a lithium ion may be mentioned for example. Ahydrogen ion is smaller than the lattice size, but is lost in water inthe form of an oxonium ion and is therefore unsuitable as ananocarbon-permeating cation. The role of such a nanocarbon-permeatingsubstance is not revealed as of now. It is however assumed that it isresponsible for altering the charge state within the nanocarbon anddisplacing impurities on the surface of the interior of the nanocarbonand inside the nanocarbon by pervading through the nanocarbon.

The content of the nanocarbon-permeating substance is preferably from0.1 to 1 mol per liter of aqueous solution for 1 g of crude nanocarbonproduct.

Next, oxidizing agents will be described. Usable oxidizing agents arenot particularly limited. Nevertheless, persulfates (persulfate ions insolution) are preferable because persulfates are alkaline and highlyactive and converted to sulfuric acid after being oxidized, which makesthem easy to be aftertreated.

Description will then be made on the pH. It is preferable that the pHranges from 6 to 14 (preferably alkaline). Reasons for suitability of aliquid being in this range is not clear. It is however assumed to beresponsible for altering the electronic state on the surface of thenanocarbon and, for a nanocarbon tube, for softening the surface of thecarbon and folding the carbon nanotube. Preferably, the pH ranges from10 to 14 for the micelle type and from 6 to 12 for the pseudomicelletype.

Description will then be made on the process for using thewater-solubilizing agent according to the invention for refiningnanocarbons (that is, a process for refining nanocarbons according tothe present invention). The process for producing high-puritynanocarbons according to the present invention will not be describedbecause the step of solubilizing, one of the steps thereof, is per sethe process for refining to be described below and the other steps ofproducing crude nanocarbons, etc. are well known at the time of filingof this application (as illustrated below).

Crude products which may be refined according to this refining processare not particularly limited. The process may effectively be applied toany crude products as obtained by any of the syntheses includingelectrical discharge (C. Journet et al., Nature 388, 756 (1997) and D.S. Bethune et al., Nature 363, 605 (1993)), laser vapor deposition (R.E. Smalley et al., Science 273, 483 (1996)), gaseous synthesis (R.Andrews et al., Chem. Phys. Lett., 303, 468, 1999), thermochemicalgaseous vapor deposition (W. Z. Li et al., Science, 274, 1701 (1996),Shinohara et al., Jpn. J. Appl. Phys. 37, 1257 (1998), plasma chemicalgaseous vapor deposition (Z. F. Ren et al., Science. 282, 1105 (1998)).

It is preferable to treat a crude product with an acid when a metalcatalyst has been used for its synthesis in order to remove the metalcatalyst before adding the crude product to the water-solubilizing agentaccording to the invention (aqueous solution for refining). For acidtreatment, a procedure as described in Japanese Unexamined PatentPublication No. 2001-26410 may be mentioned, in which a solution ofnitric acid or hydrochloric acid is used as an acid solution, beingdiluted fifty-fold with water in either case. After such acid treatment,the crude product is washed with water and filtered for subsequentdissolution.

Then the crude product containing nanocarbons (for example, carbonnanotubes) is introduced to the solubilizing agent (aqueous solution forrefining) according to the invention. Amounts to be introduced are notparticularly limited. Nevertheless, 1 to 5 g of the crude product forthe micelle type or 1 to 10 g of the crude product for the pseudomicelletype are usually introduced per liter of the aqueous solution forrefining.

After introduction, the crude product is preferably ultrasonicated firstfor about five minutes for the micelle type in order to completelydissolve the nanocarbons (carbon nanotubes, for example). It is thencompletely dissolved in six hours at a room temperature or in severalminutes with heating at 60° C.

For the pseudomicelle type, a mixture containing a pseudomicelle-formingsubstance (sodium alginate, for example), a permeator (lithiumhydroxide, for example), an oxidizing agent (sodium persulfate, forexample), nanocarbons and deionized water is thoroughly diffused anddispersed by a homogenizer, before leaving it at rest at 40° C. forabout one day. When no permeators or oxidizing agents are used, it isleft at rest at 40° C. for about one week.

After being completely dissolved, the nanocarbons (carbon nanotubes, forexample) are removed from the solution by any conventional means. Forthe micelle types, for example, the nanocarbon (for example, carbonnanotube) solution is subjected to chromatography and fractionatedaccording to sizes and water is added to the fraction of nanocarbons(carbon nanotubes, for example) to bring it below the critical micelleconcentration so that nanocarbons (carbon nanotubes, for example) may beremoved.

In addition, for the pseudomicelle type, when sodium alginate is used asa micelle-forming substance for example, 90% formic acid is used toselectively hydrolyze alginic acid so that refined nanocarbons may beremoved by filtration.

Thus, according to the invention, various impurities, for example,unwanted carbonaceous impurities such as graphite and transition metals,which may be produced depending on the processes for synthesizingnanocarbons can economically and effectively be eliminated, with aresult that high-purity nanocarbons may be obtained.

EXAMPLES

The present invention will specifically be demonstrated using Examplesbelow. The present invention is not to be limited to Examples.

Preparation of Aqueous Solution for Refining Carbon Nanotubes

Aqueous solutions for refining carbon nanotubes were prepared accordingto the formulations in Table 1 below. The diameters of microsomes in thesolutions were measured by ELS-8000 available from Otsuka ElectronicsCo. Ltd.

TABLE 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 included DSPC DMPC DPPCCHAPSO CHAPS Big-CHAP microsome 0.3 0.3 0.3 5.0 5.0 5.0 (mM) LiOH (M)0.1 0.1 0.1 0.1 0.1 0.1 (NH₄)₂S₂O₈ 0.05 0.05 0.05 0.05 0.05 0.05 (M)diameter 100 200 300 56 51 76 of microsome (nm) pH of 12 12 12 12 12 12aqueous solution

Example 1 Decatalyzation

One gram of CVD crude single-layered carbon nanotubes (purity:approximately 30%) was first decatalyzed with 1000 ml of mixed solutionof hydrochloric acid and nitric acid (0.1 M and 0.1 M) at 60° C. for 30minutes, and was then neutralized with 5 N NaOH solution and dried in atemperature-controlled bath (85° C.) for three hours.

Refining

The carbon nanotubes obtained from the above decatalyzation were addedto one liter of aqueous solution for refining carbon nanotubes andultrasonicated for about 10 minutes. This blend of the carbon nanotubesand the aqueous solution for refining carbon nanotubes was then warmedfrom the room temperature to 60° C. in a hot water bath and left forabout 10 minutes to completely dissolve the carbon nanotubes in theaqueous solution.

Recovery

A large amount of water was introduced to the carbon nanotube solutionobtained from the above refining to bring the solution below thecritical micelle concentration to obtain deposited carbon nanotubes.Recovery of the carbon nanotubes was determined to be 99.5%.

Examples 2 to 6

In a similar manner to Example 1, recovery of carbon nanotubes wasdetermined. The results are shown in Table 2.

TABLE 2 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Recovery (%) 75.1 78.2 23.8 23.545.2

Example 7

A commercially available sodium alginate (Wako Pure Chemical Industries,Ltd., weight average molecular weight 500,000) and nanocarbons (weightratio 4:1) were added to deionized water (1%, 5% and 2% in terms of theweight ratio of the sodium alginate to the deionized water). Afterthorough stirring, it was left at rest at 40° C. for about one week.Thereafter, the weight ratio of the sodium alginate, the nanocarbons andthe deionized water was diluted to about 4:1:10000 to obtain a clearaqueous solution. Then, infrared spectrum, zeta potential measurementand a transmission electron microscope were used to observe the presenceof a complex of sodium alginate and nanocarbons. Thereafter, 90% formicacid was added to this aqueous solution to selectively hydrolyze thesodium alginate and filtration was performed to obtain refinednanocarbons. The results are shown in Table 3.

TABLE 3 Single-layered multi-layered carbon carbon carbon nanotubesnanotubes nanofibers sodium alginate/  1%  5%  2% deionized water(weight ratio) recovery (%) 99.5 97.8 99.4

Example 8

Two grams of decatalyzed CVD crude single-layered carbon nanotubes(purity: approximately 30%) were added to 100 ml of an aqueous solutionat pH 12.8 containing 0.2 M lithium hydroxide, 0.1 M ammonium persulfateand the above sodium alginate (20 mg/ml) and thoroughly mixed by ahomogenizer, before leaving it at rest at 40° C. for about one day.Thereafter, insoluble impurities were removed by a centrifuge at 3000 Gand the uniformly dispersed, ink-like mixed solution was treated withformic acid (90%) at 100° C. The refined single-layered carbon nanotubeswere separated off by high-pressure filtration, thoroughly washed withdeionized water and dried at 120° C. to obtain 0.42 g of high-puritysingle-layered carbon nanotubes.

1. An aqueous nanocarbons solution comprising: nanocarbons, and anactive ingredient which is a surface active agent which is one or moreselected from the group consisting of distearoylphosphatidylcholine(DSPC), dimyristoylphosphatidylcholine (DMPC),dipalmitoylphosphatidylcholine (DPPC),3-[(3-cholamidopropyl)dimethylamino]-2-hydroxy-1-propanesulfonate(CHAPSO), 3-[(3-cholamidopropyl)dimethylamino]-propanesulfonate (CHAP)and N,N-bis (3-D-gluconamidopropyl)-cholamide, and wherein the activeingredient encapsulates the nanocarbons in globular micelles or pseudomicelles.
 2. The solution according to claim 1, which further comprisesa nanocarbon-permeating substance and an oxidizing agent and the pHranges from 6 to
 14. 3. The solution according to claim 2, wherein thenanocarbon-permeating substance is lithium ion.
 4. The solutionaccording to claim 2, wherein the oxidizing agent is a persulfate. 5.The solution according to claim 1, wherein the nanocarbons are carbonnanotubes (single- and multi-layered types and cup-stack types), carbonnanofibers or carbon nanohorns.
 6. A process for producing an aqueousnanocarbons solution comprising the step of adding a crude product to anaqueous solution containing as an active ingredient to encapsulate thenanocarbon in the crude product, a surface active agent selected fromthe group consisting of distearoylphosphatidylcholine (DSPC),dimyristoylphosphatidylcholine (DMPC), dipalmitoylphosphatidylcholine(DPPC),3-[(3-cholamidopropyl)dimethylamino]-2-hydroxy-1-propanesulfonate(CHAPSO), 3-[(3-cholamidopropyl)dimethylamino]-propanesulfonate (CHAP)and N,N-bis (3-D-gluconamidopropyl)-cholamide, and wherein the activeingredient encapsulates the nanocarbons in globular micelles or pseudomicelles.